I've done makeup in the most awkward of places: bars, benches...basketball courts!
My best advice would be to invest in what is commonly called a 'set kit'. Which is really just a fancy word for a medium sized tote bag. I use this one from Monda:
http://www.mondastudio.com/shoulder%20a … p-123.html
It was only $18 and holds everything I could need from a gig. I can easily carry 4 palettes, 8 foundations, setting spray, lashes, primers, lipsticks, etc etc...and still have room. I love this tote so much that I take it to all of my gigs. People are often very impressed by how light I pack, when in all reality I have everything I need just over my shoulder. I really utilize small traveling cases for tiny things like lip balms, eye pencils, etc.
The only other thing that I need to bring is my brush belt, which I usually stock full with obvious extras like powder, tissues, lash glue, etc...for touch ups on set. I am almost always able to find somewhere to work/lay things out: stairs, a bench, etc. However if you're at a total loss I'd just recommend having the model perch or lean against a wall while you work out of your bag.
I'd also say invest in a metal foundation palette for mixing/working off of. Worst case there is always the back of your hand too.
I also agree that water is a must and even a snack or two. I know I get pretty tired when I'm standing up for long periods/outside doing makeup.
Hope that helps! Good luck.